Hollow silica particles with the diameter of about 0.1 to about 380 μm are well known (Refer to Patent document 1 and Patent document 2). Furthermore, there has been known the method of producing hollow particles having a hard silica shell in which active silica is deposited from an aqueous solution of alkali metal silicate on a core made of a material other than silica, and the material is removed without breaking the silica shell (Refer to Patent document 3).
Furthermore, there has been known the micron-sized spherical silica particles having the core shell structure in which the external spherical surface is a shell, the central portion thereof is hollow, and the shell has a density gradient from the dense outer side toward the thin inner side (Refer to Patent document 4).
The present applicant proposed in the past to obtain composite oxide microparticles with a low refractive index and also having the size at a level of nanometers by completely coating surfaces of porous inorganic microparticles with such materials as silica (Refer to Patent document 5), and furthermore proposed to obtain silica-based hollow microparticles with a low refractive index and having the size at a level of nanometers by forming a silica coating layer on core particles of composite oxides comprising silica and inorganic oxides other than silica, removing the inorganic oxides other than silica, and coating the particles with silica (Refer to Patent document 6).
Furthermore, addition of spherical microparticles to an organic resin film is a known technique, and there has been known the fact that, with the technique, transparency of the obtained film is improved (Refer to Patent document 7).
In the films containing the silica particles or the transparent coating films as described above, there often occurs the problem that the obtained films or the transparent coating films are whitened.
Paten document 8 includes descriptions concerning the fact that, in compositions for coating containing (a) a partial condensate of an organic silicon compound comprising 100 weight portions of trialkoxysilane expressed by general formula: RSi(OR1)3 [R: a hydrocarbon group containing 1 to 6 carbon atoms, R1: an alkyl group containing 1 to 6 carbon atoms] and 20 to 130 weight portions of tetraalkoxysilane expressed by the general formula of Si(OR2)4 [R2: an alkyl group containing 1 to 6 carbon atoms; and (b) 0.05 to 200 weight portions of silica microparticles against 100 weight portions of the partial condensate (RSiO3/2+SiO2) when the trialkoxysilane is calculated as RSiO3/2 and the tetraalkoxysilane as SiO2, whitening of the coating film can be suppressed by limiting a blending quantity of silica microparticles.
Patent document 9 proposes a method of using the inorganic compound particles with the average particle diameter of 50 nm or below for preventing whitening of a transparent coating film obtained by applying an embrocation for forming a transparent coating film in which (A) a acetylacetonato-chelate compound and (B) inorganic compound microparticles are homogeneously dissolved or dispersed in a mixture solvent of water and an organic solvent to a base material and curing the embrocation.
Patent document 10 proposes a reflection-preventive film with the surface hardly damaged by frictions or the like and also with the low refractive index layer not separated, and the reflection-preventive film has a hard coat layer based on a UV cure resin mainly made of a multifunctional monomer containing two or more (metha) acryloyloxy groups in the molecule with inorganic microparticles having the average particle diameter of 0.5 to 100 nm added in the matrix, the hard coat layer formed on at least one surface of a transparent plastic film substrate, and a low refractive index layer formed on the hard coat layer and based on a matrix made of a copolymer of an organic silicon compound or a polymer thereof and an organic silicon compound or a polymer thereof with silica microparticles having the average particle diameter of 0.5 to 100 nm added therein, and the document also describes that, when using the inorganic microparticles with the average particle diameter of less than 20 nm, whitening of the hard coat layer due to scattering of light can be prevented.
As a transparent coating film containing hollow silica microparticles and an embrocation for forming the transparent coating film, for instance, Patent document 11 discloses an invention relating to a transparent film-coated substrate formed thereon, and the substrate comprises a base material and a transparent coating film provided on a surface of the substrate, and the transparent coating film has (i) a matrix containing a silicon component in turn containing a fluorine-substituted alkyl group and (ii) an outer shell layer, and also contains inorganic compound particles which are porous or have many cavities therein. The transparent coating film is also characterized that the porosity or the cavities are preserved therein. In the transparent coating film containing hollow silica microparticles and a binder as described above, however, although the low refractive index of about 1.25 to about 1.45 specific to the hollow silica microparticles can be realized, whitening of the coating film easily occurs, and there is the need for further improving the adhesiveness to the base material and the abrasion resistance.
Patent document 1: JP H06-330606A
Patent document 2: JP H07-013137A
Patent document 3: JP 2000-500113A
Patent document 4: JP H11-029318A
Patent document 5: JP H07-133105A
Patent document 6: JP 2001-233611A
Patent document 7: JP H04-348147A
Patent document 8: JP H01-306476A
Patent document 9: JP H04-247427A
Patent document 10: JP 2004-326100A
Patent document 11: JP 2002-79616A